Phthalic anhydride recovery



Jan. 5, 1937. R. RIEGLER PHTHALI ANHYDRIDE RECOVERY Filed neo. 29, 195sPatented Jan. 5, 1937 UNITED "STATES PATENT OFFICE PHTHALIO ANIIYDRIDERECGVERY *1- Riewen Riegler, Buffalo, N. Y., assgnor to National Anlineand Chemical Company, New York,.N. Y., a corporation of New YorkApplication December 29, 1933, Serial No. 704,480

3 Claims.

This invention relates to the recovery of phthalic anhydride fromgaseous and vaporous `mixtures and especially to a method'and appa- GlA4` the partial oxidation of organic compounds in accordance with onewell-known method as generally practiced, vapors of naphthalene or asubstituted naphthalene, e. g., methyl naphthalene, are mixed ,with airor another oxygen-containing gas and the mixture is passed through acatalytic converter and contacted therein with an oxidation catalystunder controlled reaction conditions. The resulting mixture of gases andvapors con- `taining a relatively small amount of phthalic anhydride andby-productA vapors is withdrawn from the converter at a temperature ofabout 350 to 550 C. and while yet at a temperature above the dew pointof the phthalicanhydride, and thereflore containing the phthalicanhydride in vapor phase, it is normally passedfto large, spacious orroom-like condensers for the condensation of the phthalic anhydridetherefrom.

In order to obtain the desired removal or Vprecipitation. of phthalicanhydride fromsuch a gas mixture, the condensation is normally effectedin a large chamber ory series of chambers, the walls of which areexposed to the outside atmosphere so that the heat of thereaction'mixture is dissipated through these walls and solid phthalic-anhydride is formed. Because of the relatively large cross-sectionalarea. of the Ycooling chamber or chambers, the gases do not flowtherethroughin a turbulent condition and hence the phthalic anhydridecrystals which are formed settle out of the gas onto the walls andbottom of the chamber from which they are periodically manuallyWithdrawn. j It is a diflicult problem to y control the ytemperature ofthe condenser in such' a manner as to effect eiiicient control ofoperations. The necessarily large size of the condensers, the inevitablefiuctuatien in the rate at which the mixture of gases and'vapors entersthe chamber and the seasonal and daily variations in at mospherictemperature 'all contribute to make it diiiicult to maintain thecondenser at constant temperature.

The phthalic anhydride recovered by this known condensation method is inthe form of long. needle-like crystals that are bulky and difficult tohandle. Furthermore, the phthalic anhydride crystals thus obtained arenot uniform in size, composition, or color. Since considerably differentconditions of temperature and saturation prevail in various parts of thechamber. there being an especially greatdiierence between. theconditions of the gases immediately after entering and their conditionvjust prior to leaving the (Cl. 26m-.123)

chamber, the phthalic anhydride crystals recovered from various parts ofthe condenser vary in size and are contaminated non-uniformly withvarious impurities. In addition, the phthalic anhydride crystals adhereto the walls of the condensing chamber so that periodically it isnecessary for workmen to enter the chamber and scale the crystals fromthe walls. As the vapors ordinarily present in the chamber areirritating to the Vskin and especially to the mucuous membranes,

the process leads to disagreeable and unhealthful working' conditions.-The cooling chamber must be exceedingly large to operate'satisfactorily and hence requires not only large `quantities ofmaterials for its construction but a large space for its placement.

An object of the present invention is to provide a process wherebyphthalic anhydride may be recovered in a simple and eiiicient mannerfrom the gaseous products of the catalytic oxidation of naphthalene andof other organic compounds while avoiding unhealthful and disagreeableworking conditions.

A further object of the invention is to provide a-process wherebyphthallc anhydride may be directly produced in the form of finelydivided particles and crystals of more uniform composition thanobtainable by the aforementioned v process.

A still further'object'lof the' invention is to eliminate theobjectionable manual scaling process above described.

A yet further object is to reduce the amount of placement spacenecessary for condensers and to reduce the vamount of materialsnecessary in their construction.

Further objects will be evident from the following description.-

In accordance with the present invention phthalic anhydride, isprecipitated and recovered from a vapor, for instance the mixtureobtained by the vapor phase. catalytic, partial oxidation of-naphthaleneby air, by passing the vapor through an externally cooled condenserunder definitely controlled conditions ci' temperature and having a gaspassage of relatively small cross-se`ction so that a condition ofturbulent ow oi gases passing therethrough 1s maintained. By

p arranging the condenser so that the gases pass therethrough in acondition of violent agitation or turbulence, af far greatereffectiveness of cooling surface is attainable and hence much lesscooling surface'is necessary. An elongated required. The mechanicalscraper serves to keep the walls of the condenser clean and free fromphthalic anhydride so that a maximum eiilciency of heat transfer'between the gases and the cooling wall is maintained. 'Ihe scraperfurthermore serves as aconveyor to impel the separated phthalicanhydride toward suitable collecting apparatus.

In the event the phthalic anhydride is condensed from a gaseous mixturesuch as that obtained by a catalytic oxidation process,.the residualgases after the condensation will ordinarily contain phthalic anhydridein the form of` fine dust. The gasescontaining this dust are passedthrough' a cleaning device, such as a settling chamber or bag lter, forthe purpose of removing the phthalic anhydride dust.

' By the use of the present invention highly eillcient cooling isobtained and a much shorter time is necessary to effect the phthalicanhydride condensation. The recovery apparatus, therefore,'may be muchsmaller than the condensing chamber of the type previously described.Furthermore, with the smaller apparatus more uniform conditions ofcondensation prevail so that a product of greater uniformity is obtainedby the condensation. In addition the separated product in its passagethrough the condenser is caused to become intimately mixed so that avery uniform product is flnally obtained.

The condenser employed in the present process may be cooled by anysuitable cooling uid. Good results are obtained using a water-jacketedor brine jacketed", tubular condenser. With this type of condenser ahigh rate of heat transfer is obtainable between the cooling fluid andthe condenser wall andbyusing an eflicient scraping device, for examplea continuously operated hellcoidal screw, heat transfer between thecondenser wall and the hot vapors may be maintained at a highefficiency.

The condenser may be constructed of any suitable material of good heatconductivity that is not seriously corroded by the vapors being treated.Ordinary steel or stainless steel fulfills these requirements but anyother suitable material may be employed.

In order that a better understanding of my invention may be had,l thefollowing description is set forth of a specific embodiment, which isillustrated in the accompanying drawing.

Fig. 1 shows diagrammatically apparatus for the preparation of phthalicanhydride by the catalytic oxidation of naphthalene with air andembodies one form of condensation apparatus of the present invention;

Fig. 2 is a fragmentary elevation of the cleaner of Fig. 1 andindicates, also diagrammatically, the relation of condenser and cleanerinl the described embodiment.

With especial reference to Fig.A 1, the numeral I indicates aconventional naphthalene vaporizer having a naphthalene inlet 2 and airinlet 3. The vaporizer I may be provided with a steam jacket I* asshown. The vaporizer is connected by pipe 4 to the catalytic converter 6which may be a converter of the ordinary type ,employed in the catalyticoxidation of naphthalene and may be provided with a temperatureregulating jacket and coolers 5*, as indicated. An auxiliary air inlet 6is shown for regulating the ratio ofl air and naphthalene vapor passingto the converter. The catalytic converter l is connected by means ofpipe 1 to a cooler l, which inturn is connected by means of a jacketedA(or lagged) pipe i to condenser I0.

The condenser I0, as shown, isv provided with a cooling packet I Ilnhaving an inlet II and outlet I2 for the passage of a cooling fluidtherethrough. The condenser I0 is provided with a screw conveyor orscraper I3, which may be driven in any suitable manner, for example byan electric motor Il, which may be mounted at either end of the scraper.As shown in the drawing, the scraper I3 is helicoidal in form anddefines a spiral path for gases passing through the condenser; it isprovided with a central-supporting shaft I5. At the end of the condenseradjacent the gas inlet pipe 9, the scraper has no central shaft, therebypermitting direct longitudinal as well as spiral flow of gases throughthe condenser in this vicinity. This design also provides more easyaccess to the interior of the condenser for cleaning at this end. Theshaft I5, which carries scraper I3. is suitably mounted in bushings I 6.A removable or transparent panel I1 may provide an' observation port forinspection of the condenser interior. As previously noted, the condensermay be constructed of any vsuitable material, and steel or stainlesssteel, because of their good heat conductivity and because they arerelatively little affected by erosion and by the corrosive action of thegases, are especially suitable. The size of the condenserwill dependupon the volume of gas to bel treated but, as illustrative, a condenserhaving an inner diameter of a foot and a length of 20 feet may beeffective with suitable regulation of cooling fluid, say water enteringat about 0 to 10 C., for the condensation of phthalic anhydride from thegases resulting from the catalytic oxidation of pounds naphthalene perhour. The condenser I0 is shown in the drawing arranged horizontally. Itmay, however, be arranged either in an inclined or vertical position andin such case it is desirable that the' flow of gas and phthalicanhydride be generally downward rather than upward through thecondenser. The end vof the condenser I0, remote from the gas inlet,conducts immediately into a dust separator I8.

The separator I8 is shown as a bag lter provided with bags IS andsuitable bag shaking mechanisms I9, The separator I8 is preferablyseparated by means of a baille IB* into two chambers each connected withpipe 20 through a valve The bag shaking mechanisms I9a are so arrangedthat the bags of each section or chamber of the apparatus are shakenalternately, the valves 20a. being controlled so that the gases arepassed through that section which is not being shaken. Conduit 20 leadsto a blower and vents to the atmosphere or to scrubbing apparatus. Thelower portion 2 I of separator I8 is hoppershaped and is provided with ascrew conveyor 22, more clearly shown in Fig. 2. At one end of thishopper there is provided a rotary valve 23 for withdrawing solidproducts from the hopper without removal of gases. In Fig. 2 thecondenser I0 is indicated as positioned at the same end of the hopper asthe rotary valve 23. It mayof course, be 'positioned wherever desiredsince phthalic anhydride admitted from the condenser will be conveyedtogether with that separated in the -bag fllter Il, byA means of screwconveyor 22 to the removal end of the hopper; Conveyor 22 may becontinuously or intermittently operated, as by a motor 24.' A container25 for collecting phthalicfanhydride is placed beneath discharge valve23.*

thalene vapors. The resultant gas and vaporl aoeaoie suitable' heatingJackets and/or lagging (not shown) are provided to maintain necessarytemperature in the various pieces of apparatus, piping, etc., and/or toreduce loss of heat,as is customary in the art.

The operation of the apparatus is as follows: Naphthalene and air aresupplied to vaporizer i by means of inlets 2 and 3, the naphthalenebeing maintained in molten condition and the air being bubbled uptherethrough to carry oi naphmixture is withdrawn and passed by means of4- pipe t to the catalytic converter 5; auxiliary air vthe temperatureis reduced to around 120 to 135 may be introduced through pipe 6, i!desired, the

air and naphthalene entering the converter being regulated to a ratioof25 to 35 parts by weight of air-to each part by weight of naphthalene.

In the catalytic converter the elevated temperature is maintained by theheat of the reaction, a cooling uid being provided around the /tubes ofthe converter to prevent excessive rise of'temperature therein. Thiscooling fluid may maintain the' proper temperature in known manner-, -Afor` example a liquid that boils at the required temperature may beused, and maintained at the boiling point by means oi the4 hot reactionmixture within the converter and the coolers te.'

which condense thevapors of the cooling liquid and return thecondensate. Ordinarily the gases will leave the converter at atemperature of' around 450 C.

Products oi the reaction pass from the converter t by means of pipe 7iinto cooler d where Y C.,- that is, to a temperature slightly, above thedevT point of phthalic anhydride in the gas mixture.

The cooled gases and vapors pass by means oi a pipe it into thecondenser it, entering the condenser while still slightly above the dewpoint of phthalic anhydride. A cooling uid, for example cold water orbrine at say 010 C., is passed through inlet il into' the jacket idf* ofcondenser iii and isl withdrawn at outlet it. l The walls of thecondenser. are thus maintained at a temperature well below the meltingpoint of phthalic anhydride. By the passage ci the gas mixture intocontact with the cooled walls oi' the condenser, phthalic anhydride iscaused to sepalrate out in the form of fine crystals;

Motor it is operated to drive the scraper it so that crystals separatingout on the walls of the condenser will be scraped therefrom and so thatthe phthalic anhydride which collects at the bottom of the condenser isimpelled, in a generally concurrent direction with respect to gas iow,toward the outlet end of the condenser, whence it passes into -thehopper oi cleaner IB.

In order to avoid clogging of the conveyor with phthalic anhydride atthat portion of the'condenser immediately surrounding. the gas inlet,

Vwhere the gases are brought into contact vwith the shaft is omittedfrom this portion of the conthe scraper before cooling has becomeeffective.

veyor so as to avoid obstruction of the gas passage. Satisfactory.operation is obtainedgwith a condenser having the shaft omitted fromonly the first two or three conveyor iiights. l

The gases areA cooled in thev condenser lil .to a temperatureof about 30to 40 C. and at this temperature are relatively free from phthalic .-.nanhydride in vapor form.' They yet contain 15% to 5`%` bf their-original phthaiic *anhydridev content, however, in the form of a moreor less fine dust, which because of its flneness, does not settle out inthe condenser. The gases containing the phthaiic anhydride dust arepassed up through the bag filter I8 passing alternately through eachsection. Ordinarily thezcapacity of the bagtilter will be sufiicient sothat the gases may pass through it in a non-turbulent condition. In thismanner the phthalicl anhydride dust is ltered from the gases; the gasesare expelled through outlet 2i).v During the passage of the gasesthrough the lter bags I9 of each section of separator it, the bags ofthe other section are shaken by shaking mechanism i9 causing separatedphthalic anhydride to fall into the hopper 2i. At the bottom of thishopper screw conveyor 22 is operated in any suitable manner to impel theseparated dust toward the rotating valve 23. The dust collected by thebag iilter iii and the ne, crystalline powder collected in the condenserit,

' pass together towards the outlet valve 2t; the

valve may be operated in any suitable manner,

for example by the same drive means as conveyor tinto permit passage ofthe phthalic anhydride to a container 2d without the vescape of gas. Itwill be obvious that. if desired, the ner dust recovered from the ltermay be collected apart from the powder obtained in the condenser.

I claim: 1. The method oi cooling phthalic anhydride I vapors andpreparing phthalic anhydride in the form of a solid, crystallineproduct, which cornprises passing phthallc anhydride vapors intoy ascrew conveyor the walls of which are maintained at a temperaturesubstantially below the condensation temperature of phthalic anhydrideand' adjusting the rate of ow of vapors so that the Vvapors aremaintainedin a state of turbulence as they contact with the cooled wallsof the conveyor, and operating the .screw conveyor so as to continuouslyVscrape resultant crystallized phthalic anhydride from said walls.

2. In the preparation of phthalic anhydride by the catalytic oxidationof naphthalene, the steps which comprise cooling the hot reaction gasesand vapors to .a temperature slightly above the dew vpoint of phthalicanhydride, passing the reaction gases and vapors at a temperatureslightly above the dew point of pnthaiic anhydride into a tubularIcondenser'and through the condenser as a turbulent stream so as toprovide lviolent agitation of the gases and vapors in contact withvthecooled portions of said tubular condenser and condense phthalicanhydride in the form of solid crystals, withdrawing the cooled gasescontaining phthalic anhydridedust from the condenser and reducing theirvelocity, and subjecting the gases in non-turbulent condition to acleaning treatment for the removal of the phthalic anhydride

